Casting process

ABSTRACT

A process and apparatus for flattening a metal sheet and casting a coating comprising supporting a sheet on a support having a substantially air impervious bottom and a substantially air impervious wall which enclose a region having an open top and containing an air pervious supporting means, and removing air from the region of the air pervious supporting means until the air pressure above the sheet exceeds the air pressure within the region and forces the sheet against the pervious support substantially free from undulations and then casting a coating over at least part of the sheet while it is substantially free from undulations and maintaining the sheet free from undulations until the coating is a solid.

United States Patent 1 1 Crowe [45] Apr. 24, 1973 CASTING PROCESS [75] Inventor: Gordon Appling Crowe, Ann Arbor,

Mich.

[73] Assignee: North American Urethanes, Inc.,

Upsilanti, Mich.

[22] Filed: Mar. 1', 1971 21 Appl.No.:119,522

29/527.4; 117/132 R, 132 A, 132 B, 132 BE, 132 C, 119, 61; 264/219, 90, 259, 316, 92; 118/500, 50, 415; 72/60 3,434,181 3/1969 Benzies 264/90 X 3,461,838 8/1969 Nelson, Jr. et al ..118/50 3,492,384 1/1970 Matthews et a1. ..264/316 X Primary Examiner-Charles W. Lanham Assistant ExaminerV. A. Dipalma Attorney-Robert M. Phipps et al.

[ 5 7 ABSTRACT A'process and apparatus for flattening a metal sheet and casting a coating comprising supporting a sheet on a support having a substantially air impervious bottom and a substantially air impervious wall which enclose a region having an open top and containing an air pervious supporting means, and removing air from the region of the air pervious supporting means until the air pressure above the sheet exceeds the air pressure within the region and forces the sheet against the pervious support substantially free from undulations and [56] References Cited then casting a coating over at least part of the sheet UNITED STATES PATENTS while it is substantially free from undulations and maintaining the sheet free from undulations until the 2,470,111 5/1949 Rubissow ..117/119 coating is a S01i 2,814,077 ll/l957 Moncrieff ..18/56 3,180,011 4/1965 Heuer ..29/l57.3 4 Claims, 3 Drawing Figures 3 6 4 3 K K r 5 Patented April 24, 1973 VIII"""pbapplpp'lralnl- CASTING PROCESS This invention relates generally to the casting of films and more particularly to an improved method of casting relatively thin films of substantially uniform thickness wherein a liquid is spread over a surface and allowed to solidify.

Various methods and apparatus have been suggested for preparing films and coatings by spreading a material over a surface while it is liquid and maintaining it in its spread state until it solidifies. The resulting coating may be left on the surface as a coating or it may be removed and used as a self-sustaining film. For example, selfsustaining sheets of polyurethane are often formed by casting a liquid reaction mixture over the surface of a relatively thin metal sheet coated with a mold release agent and, after chemical reaction and solidification of the mixture, removing the resulting film from the metal sheet. In those instances where it is desirable to produce a coated sheet, mold release agents are not used and the film is not removed from the sheet.

Of the various processes suggested for making such products the one most often used commercially, particularly for preparing polyurethane films, involves pouring a liquid castable reaction mixture over the surface of the sheet while it lies on the heated surface of a casting table and, after the resulting film has solidified, removing the metal sheet from the casting table. More specifically, in preparing an adherent coating, a relatively thin metal sheet may be sand blasted, coated with a bonding agent and clamped or otherwise securely fastened to the surface of a heated casting table. Metal bars are placed on the sheet to form picture frame type mold walls enclosing the area of the sheet to be coated. The correct volume of liquid urethane reaction mixture or other suitable solidifiable mixture is poured over the surface of the enclosed area while it is still sufficiently fluid to seek a common level. After solidification the metal sheet is removed from the table and the film is removed therefrom.

Such a process is relatively simple and has many advantages but has the disadvantage of often not producing a film of uniform thickness particularly when the metal sheet and the film cast thereon are relatively thin. Even the highest quality stretcher-levelled thin sheet metal is not truly flat. It usually has many bends, bulges and ripples which because of its inherent springiness will remain unless held firmly against a. flat surface. Frequently, such a sheet will have waves or undulations of a depth equal to or greater than the thickness of the film to be cast on the surface of the sheet. Consequently, the liquid layer poured over the surface will flow into low spots and produce puddles here and there and leave thin layers of liquid or no liquid at all at the high spots. Attempts have been made to hold the sheet flat with the bars used as a mold frame but the sheet still has a sinuous cross-section in the area enclosed by the bars. Weights have also been placed over the entire sheet with only small points spaced here and there touching the metal sheet within the area enclosed by the bars, but the coating has a void for each point which must later be filled by patching. The product has a patched appearance.

It is therefore an object of this invention to provide a process for casting films which is devoid of the disadvantages of the prior art processes. Another object of the invention is to provide an improved process for casting films prepared by shaping a liquid and effecting solidification thereof. Still another object of the invention is to provide an apparatus for maintaining a metalsheet having a sinuous cross-section flat while a liquid is cast and solidified thereon. A further object of the invention is to provide an apparatus of simple and inexpensive constructionadapted to flatten a relatively thin flexible metal sheet having a sinuous cross-section. A more specific object of the invention is to provide a simple but effective process and apparatus for casting relatively thin films of synthetic resins or the like of substantially uniform thickness over a metal sheet having a surface which under normal conditions is not perfectly flat.

Other objects will become apparent from the following description with reference to the accompanying drawing wherein FIG. 1 is a plan view, partially fragmentary, of one embodiment of the invention; I

FIG. 2 is a cross-section taken along the line IIII of FIG. 1; and

FIG. 3 is an exaggerated illustration of the edge of a metal sheet having a sinuous cross-section adapted to be flattened in accordance with this invention.

The objects of the invention are accomplished, generally speaking, by providing an apparatus for supporting a sheet having a sinuous cross-section and evacuating the region below the sheet until the sheet because of the difference in pressure above and below the sheet is pressed against the support in a flattened condition. In other words, the sheet is supported to prevent buckling downwardly as air is withdrawn and is consequently straightened or leveled by atmospheric pressure which forces it firmly against the various spaced points of the support which are in contact with the underside of the sheet.

In a preferred embodiment of the invention, a thin metal sheet is sand blasted or otherwise cleaned by conventional methods prior to coating. A gasket is adhesively secured around the periphery of the underside of the sheet leaving an open region therebetween. A piece of expanded metal which has not been flattened after expansion and has been cut to dimensions corresponding substantially to the dimensions of the open region is placed on the surface of a level casting table. The sheet is then placed over the casting table with the gasket about the expanded metal and resting on the casting table. A hole is drilled in one corner of the sheet to provide for passage of one end of a tube into the open region under the expanded sheet. The other end of the tube is attached to the suction side of a blower, to a vacuum pump or a combination thereof. Air is withdrawn from the region under the sheet until the air pressure above the sheet is sufficiently greater than that within the region to firmly press the sheet against the upper most points of the expanded metal and to flatten the surface of the sheet by removal of any undulations therein. A coating is then cast over the surface of the flattened sheet by pouring a liquid thereover and continuing to maintain the vacuum until the liquid has solidified.

Referring now to the drawing, a substantially air impervious rubber gasket 5 is adhesively bound around the periphery of the underside of a flexible relatively thin steel sheet 2. One-half inch wide by three-sixteenths inch thick foam rubber weather stripping of the type available commercially for 4 weather stripping doors and windows of a residence may be used to form gasket 5. Most stripping has one surface coated with a pressure sensitive adhesive which may be used to bind the stripping to the sheet. A sheet of unflattened diamond patterned expanded metal 4 having a maximum thickness of about one-eighth inch and a length and width about equal to those of the region enclosed by gasket is placed on the surface of a heated substantially level casting table 1. The assembly of sheet 2 and gasket 5 are positioned over table 1 with gasket 5 surrounding the expanded metal sheet 4'. Steel rods 3 are positioned on the exposed surface of sheet 2 in the desired configuration. and dimensions. The weight of sheet 2 and rods 3 compress gasket 5 until it is about equal in thickness to the thickness of sheet 4. One end of a suitable tubing is passed through opening 7 into the region of expanded metal sheet 4 and is connected at its other end to a means for removing air from the region, such as an aspirator, vacuum pump, suction fan or the like. Air is withdrawn from the said region until the pressure above sheet 2 is sufficiently greater than that below sheet 2 to firmly press sheet 2 against sheet 4, remove any undulations therein resulting from its sinuous cross-section and thereby flatten the expanded surface of sheet 2. While maintaining less than atmospher- 3. After solidification of the polyurethane, air is permitt'e d to flow into the space defined by gasket 5, the rods 3 are removed and sheet 2 is removed from the casting table 1. If sheet 2 is used as a casting surface for a film which is to be removed therefrom and used as a self-sustaining film, sheet 2 and the inner walls of rods 3 are coated with a suitable mold release before the reaction mixture is poured over sheet 2 to facilitate separation of the cast film from sheet 2. Any suitable mold release known in the art may be used such as a wax, a silicone or one of the mold release agents disclosed in the Saunders and Frisch book, identified more specifically hereinafter, if the film is to be removed from the metal sheet. Any' known suitable bonding agent may be used in preparing an adherent coating on a metal sheet.

The process and apparatus provided by the invention may be used to cast any liquid which will solidify into a product which is a solid at C., such as, for example, molten paraffin-wax, beeswax or other wax, a liquid urethane reaction mixture, any molten resinous material or the like. Suitable mixtures for casting self-sustaining polyurethane films or adherent coatings on a sheet are disclosed, for example, in the book entitled Polyurethanes: Chemistry and Technology by Saunders and F risch, published by lnterscience Publishers, Copyright 1962, Library of Congress Card Number 62-18932. Any of the formulations disclosed in this book for making coatings or castings may be used. Any other liquid which when poured over the surface of the sheet is sufficiently fluid to seek a common level on a flat surface and will solidify into a film which is solid at 20C. may be used.

While best results have been obtained so far with an expanded metal sheet any other support which will not block flow of air from under the sheet 2 and will provide sufficient support to prevent buckling or collapse of sheet 2- when air is withdrawn from under sheet 2 may be used in combination with an air impervious wall I thereabout. For example, a labyrinth formed by positioning metal bars or strips of a thickness substantially equal to the thickness of gasket 5 when it is supporting sheet 2 may be used provided passageways are availa- 'ble for flow of air from under sheet 2. Other porous materials such as a sheet of open celled rigid polyurethane foam or styrene foam may also be used. Usually the thickness of gasket 5 and of the air pervious material cooperating therewith to support sheet 2 is relatively thin and often a quarter of an inch or less. Expanded metal sheets are fabricated by first cutting a plurality of slits in a sheet and then stretching the metal to separate the walls of the slits and form a polygonal shaped opening similar to that shown in the drawing. The strips of metal between the openings do not all lie in the same plane so sometimes the product is pressed to flatten it before it is used for a screen, a cat-walk,

steps or the like. However, since some thickness is desired'and blockage against air flow is to be avoided, unpressed or unflattened expanded metal is used in this invention. A suitable fitting or plug may be used to provide an air impervious fitting between the tubing and opening 7. Although it is preferred to adhesively bind gasket 5 to the metal sheet, in some instances the adhesive can be omitted.

The invention is primarily advantageous for coating sheet metal, such as, steel, aluminum, brass and the like, but it may also be practiced to coat any other flexif ble substrate which has a sinuous cross-section, such as, sheets of various plastic or resinous materials.

In the preferred embodiment of the invention,

described above, air may be first withdrawn from under sheet 2 with the suction side of a fan type blower such as a portable vacuum cleaner or similar apparatus capable of withdrawing a relatively large volume of air rapidly. This facilitates seating the gasket against the surface of the casting table and sealing the region enclosed by the gasket. After the gasket is seated against the table, a vacuum pump capable of removing substantially all of the air from under sheet 2 may be cut in to continue withdrawing air until the sheet is firmly pressed against the expanded metal sheet and any undulations have been removed. One arrangement of apparatus suitable for this embodiment employs a twoway valve in the tubing extending from hole 7 with a vacuum cleaner and a vacuum pump attached through suitable tubing to the valve.

The invention is applicable to the-coating of relatively large sheets as well as smaller ones. Sheet steel 4 X 8 feet and about one-sixteenth inch thick and larger may be coated by the process with coatings as thin as onesixteenth inch or less. The formulations and processing steps in Chapter X, Section IV, of the Saunders and Frisch book, Part II, may be used in combination with the flattening step of this invention.

Although the invention has been described in detail for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of casting a coating over at least part of a flexible sheet structure having a sinuous cross-section which comprises 1. supporting the sheet on a support having a substantially air impervious bottom and a substantially air impervious wall enclosing a region having an open top and containing an air pervious supporting means which defines a flat support plane at said open top,

2. removing air from the region of the air pervious supporting means until the air pressure above the sheet exceeds the air pressure under the sheet and within the air pervious region and forces the sheet against the air pervious support substantially free from undulations,

3. placing a retaining wall on top of and in contact with the sheet and enclosing an area,

. casting a coating over at least part of the sheet and within the retaining wall while it is substantially free from undulations, and

5. maintaining the sheet free from undulations until the coating is solid.

2. The method of claim 1 wherein the coating is a polyurethane and the sheet is supported over a heated casting table.

3. The method of claim 1 wherein the air pervious support is unflattened, expanded metal.

4. A method of casting a film of substantially uniform thickness over a flexible metal sheet having a sinuous cross-section which comprises adhesively binding a strip of gasketing material about the periphery of one side of the sheet, placing on a support an expanded metal sheet of approximately the same thickness as that of the said strip and having a width and length substantially equal to the width and length of the region enclosed by the strip, said expanded metal sheet provid ing a plurality of points spaced upwardly above the surface a distance substantially equal to the height of the gasketing material whereby a substantially flat cover over the open top would be supported by the gasketing material and the points, placing the flexible sheet over the expanded metal sheet with the gasket surrounding the expanded metal sheet, withdrawing air from the region of the expanded metal sheet until the flexible sheet is substantially free from undulations and maintaining it in such a position while pouring a liquid over the exposed surface of the flexible sheet and within a retaining wall and allowing the liquid to solidify into a coating over the surface of the flexible sheet, and thereafter permitting air to flow into said region. 

1. The method of casting a coating over at least part of a flexible sheet structure having a sinuous cross-section which comprises
 1. supporting the sheet on a support having a substantially air impervious bottom and a substantially air impervious wall enclosing a region having an open top and containing an air pervious supporting means which defines a flat support plane at said open top,
 2. removing air from the region of the air pervious supporting means until the air pressure above the sheet exceeds the air pressure under the sheet and within the air pervious region and fOrces the sheet against the air pervious support substantially free from undulations,
 3. placing a retaining wall on top of and in contact with the sheet and enclosing an area,
 4. casting a coating over at least part of the sheet and within the retaining wall while it is substantially free from undulations, and
 5. maintaining the sheet free from undulations until the coating is solid.
 2. removing air from the region of the air pervious supporting means until the air pressure above the sheet exceeds the air pressure under the sheet and within the air pervious region and fOrces the sheet against the air pervious support substantially free from undulations,
 2. The method of claim 1 wherein the coating is a polyurethane and the sheet is supported over a heated casting table.
 3. The method of claim 1 wherein the air pervious support is unflattened, expanded metal.
 3. placing a retaining wall on top of and in contact with the sheet and enclosing an area,
 4. casting a coating over at least part of the sheet and within the retaining wall while it is substantially free from undulations, and
 4. A method of casting a film of substantially uniform thickness over a flexible metal sheet having a sinuous cross-section which comprises adhesively binding a strip of gasketing material about the periphery of one side of the sheet, placing on a support an expanded metal sheet of approximately the same thickness as that of the said strip and having a width and length substantially equal to the width and length of the region enclosed by the strip, said expanded metal sheet providing a plurality of points spaced upwardly above the surface a distance substantially equal to the height of the gasketing material whereby a substantially flat cover over the open top would be supported by the gasketing material and the points, placing the flexible sheet over the expanded metal sheet with the gasket surrounding the expanded metal sheet, withdrawing air from the region of the expanded metal sheet until the flexible sheet is substantially free from undulations and maintaining it in such a position while pouring a liquid over the exposed surface of the flexible sheet and within a retaining wall and allowing the liquid to solidify into a coating over the surface of the flexible sheet, and thereafter permitting air to flow into said region.
 5. maintaining the sheet free from undulations until the coating is solid. 